It is well known that the aesthetic desirability, corrosion resistance and paint adhesion properties of an aluminum surface may be improved by forming a chromate conversion coating by contacting the surface with an aqueous conversion coating composition containing hexavalent chromium ions, phosphate ions and fluoride ions. By way of illustration, such coatings are disclosed in U.S. Pat. Nos. 2,438,877; 2,928,763 and 3,377,212 and in British Pat. No. 891,910. However, there has been growing concern in recent years about pollution of rivers and waterways, and attention has been directed to reducing or eliminating harmful materials from the effluent of industrial processes. Hexavalent chromium can cause problems if discharged into waterways because of its strongly oxidizing character. As a result, conventional chromate conversion coating processes have required extensive waste treatment procedures to eliminate possible harmful effects resulting from the discharge of hexavalent chromium. This necessarily results in increased costs, and as a result attention has been given to developing alternative processes for improving the characteristics of aluminum surfaces which employ no hexavalent chromium.
Recently, attempts have been made to produce chromate-free coatings. By way of illustration, U.S. Pat. No. 3,682,713 discloses an aqueous aluminum coating solution containing a complex fluoride of boron, titanium, zirconium or iron and an oxidizing agent such as sodium nitrobenzene sulfonate or nitrate. As a further example, co-pending U.S. application Ser. No. 755,922 discloses an aqueous aluminum coating solution containing zirconium and/or titanium, fluoride and phosphate. However, it is desirable from an environmental standpoint to produce a coating solution that does not contain phosphates, nitrates or sulfonates. Such a coating solution is disclosed, for example, in U.S. Pat. No. 3,912,548 which discloses inter alia a metal coating solution comprising polyacrylic acid and ammonium fluozirconate.
It has now been found that novel aluminum coating compositions consisting essentially of (a) polyacrylic acid or esters thereof and (b) an acid selected from the group consisting of H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6, and H.sub.2 SiF.sub.6 are particularly effective in providing improved corrosion resistance and paint adhesion for the surface of the aluminum.